In the description that follows references are made to certain compounds, devices and methods. These references should not necessarily be construed as an admission that such compounds, devices and methods qualify as prior art under the applicable statutory provisions.
With the increasing importance of batteries for a wide variety of uses, ranging from portable electronics to power supply devices for spacecraft, there is a long-felt need for new materials with higher energy densities.
The energy density of a material can be quantified by measuring the amount of electron-donating atoms that can reversibly react with the material. One way of obtaining such a measurement is by setting up an electrochemical cell. The cell comprises a container housing an electrolyte, one electrode made of the electron-donating material (e.g.xe2x80x94an alkali metal), another electrode made of the material whose capacity is being measured (e.g.xe2x80x94a silicon nanostructure-based material), and an electrical circuit connected to the electrodes. Atoms of the electron-donating material undergo an oxidation reaction to form ions of the donating material, and free electrons. These ions are absorbed by the opposite electrode, and the free electrons travel through the electrical circuit. Since the number of electrons xe2x80x9cgiven awayxe2x80x9d by each atom of the electron-donating material is known, by measuring the number of electrons transferred through the electrical circuit, the number of ions transferred to the material being investigated can be determined. This quantity is the specific capacity of the material, and can be expressed as milliampere-hours per gram of the material. For example, the maximum specific (reversible) capacity of graphite to accept lithium is reported to be approximately 372 mAh/g. Because one lithium ion is transferred to the graphite electrode for every electron released, the specific capacity can be expressed in terms of the stoichiometry of the electrode material. For graphite, the saturation phase can be characterized as LiC6 with Li ions residing between the graphene layers. See, for example, (M. Winter et al., Insertion Electrode Materials for Rechargeable Lithium Batteries, Advanced Materials, Vol. 10, 10, xe2x80x9c725-762xe2x80x9d,1998; and J. R. Dahn et al., Mechanisms for Lithium Insertion in Carbonaceous Materials, Science, volume 270, Oct. 27, 1995.
Lithium intercalated graphite and other carbonaceous materials are commercially used as electrodes for advanced Li-ion batteries. See, for example, M. S. Whittingham, editor, Recent Advances in rechargeable Li Batteries, Solid State Ionics, volumes 3 and 4, number 69, 1994; G. Pistoria, Lithium Batteries: New Materials, Development and Perspectives, Elsevier, 1994. The energy capacities of these conventional battery materials are partially limited by the LiC6 Li saturation concentration in graphite (equivalent to 372 mAh/g).
In order to increase the capacities of electrode materials other carbon based-materials have attracted attention as potential electrode materials. Disordered carbon (soft and hard carbon) materials show reversible lithium storage capacities higher than that obtained from graphite (see, for example, J. R. Dahn et al., Mechanisms for Lithium Insertion in Carbonaceous Materials, Science, volume 270, Oct. 27, 1995). Single wall carbon nanotube bundles have a large reversible Li storage capacity of 1000 mAh/g, but at a large voltage hysteresis.
Lithium alloys have been investigated as possible anode materials for Li-based batteries. Si and Ge are known to form Li-rich alloys with compositions up to Li22Si5 or Li22Ge5. They have been investigated for application in high temperature molten salt batteries (see, for example, R. N. Seefurth and R. A. Sharma, Investigation of lithium utilization from a lithium-silicon electrode, J. Electrochem. Soc., Vol. 124, No. 8, 1207-1214, 1977). However, electrochemcial reaction of Li with Si or Ge is only possible at high temperatures (higher than 350xc2x0 C.).
Pyrolysis of carbon and silicon-containing precursors has yielded materials with enhanced Li storage capacity (500-600 mAh/g) (see, e.g.xe2x80x94Carbonaceous materials containing silicon as anodes for lithium-ion cells, Mat. Res. Soc. Proc., Vol. 393, page 305-313, 1995).
It would be desirable to develop other materials having improved energy storage capacities and energy transfer properties. There exists a long-felt, but so far unfulfilled need, for a material having such properties. There exists a need for a material having improved properties that make it useful in battery electrodes and other high energy applications.
These and other objects are attained according to the principles of the present invention.
One aspect of the present invention includes a material comprising a nanostructure that can reversibly react with foreign species. The material having a reversible capacity of at least 900 mAh/g.
Another aspect of the present invention includes a material comprising silicon rod or wire-like nanostructures and intercalted lithium, the material having a reversible capacity of at least 900 mAh/g.
A further aspect of the present invention includes a germanium-based material comprising a germanium and germanium oxide nanostructure. The material having a reversible capacity of at least 100 mAh/g.
In another aspect of the present invention, an article comprising an electrically conductive substrate, and a film deposited on the substrate which comprises any of the above-described materials. The article may take the form of an electrode for a battery.